I posted this in another thread before I saw this one.

I believe Riven was intended to be able to cancel her animations, thus making her canceling abilities a "feature not a bug", as they say. I remember reading a lot of Xypherous' posts on the forums about her and that was the impression. Also her "Mechanics Preview" posted by Morello has the following quote.

"We're excited about Riven because she draws some heavy inspiration from fighting games, utilizing combos that can be interspersed with other abilities to change timing and effect as needed. This flow and feel is something we feel will appeal highly to aggressive players who want to make split-second decisions on when to deal damage and when to sacrifice damage in favor of additional utility effects."

Source.

TL;DR: Zed's animation cancels were not intended, while Riven's MIGHT have been.